To form images, there is a known technique, particularly in the field of X- or gamma ray imaging, of using a parallelepiped detection device with two main directions, generally of the matrix type. The two main directions conventionally define a detection plane, in which the detector can be used to locate the point at which the incident radiation interacts with the detector. It is additionally known for such detectors to be used to form digital images, i.e. images encoded in an information-processing bit sequence.
Such a detector generally includes a material that interacts with said incident radiation by releasing mobile electrical charges, and electrodes between which is defined an electric field, that induces the displacement of the released mobile electrical charges. Among these electrodes can be distinguished in general a unitary cathode, forming an equipotential set on the detection plane, and one or more anodes, constituted by a plurality of juxtaposed points or pixels forming a matrix in the detection plane. These electrodes therefore collect said charges constituting the detection signal, and are connected to measurement gages. They are then referred to as collecting electrodes.
Some constituent materials of such detectors do not have good charge carrying properties. Thus, beyond a certain thickness, the charges either do not reach said collecting electrodes, or reach them in too small a number. The signal collected is then either zero, or too weak to be exploited. As a result, the effective thickness of the detector material is limited by the charge carrying properties of said material. Detection efficiency, defined as being the ratio between the number of photons detected and the number of incident photons, is in fact directly dependent on the thickness of said detector material. This efficiency is therefore also limited by the charge carrying properties of the detector material.
Furthermore, it is known that the displacement of a charge carrier cloud, in the vicinity of an electrically insulated electrode, generates by capacitive effect, a charge known as an induced charge in said electrode. This type of electrode is commonly known as a non-collecting electrode. The use of such a non-collecting electrode is known from the prior art.
A detection device has for example been described in the document FR-2 887 993 constituted by a detector material, whereof the opposite faces are connected to a cathode and to an anode network respectively, the latter being constituted by coplanar anodes arranged in lines, known as anode lines. A layer of electric insulator is placed between the anode plane and a network of non-collecting electrodes, in such a way that the collection of a charge on the anode induces a charge in the non-collecting electrodes situated in proximity. In doing this, said device enables the deduction of the position on X and Y, in other words in the two main directions of the detection plane, of the collection in the anode plane, and which also corresponds to the position on X and Y of the interaction with the incident radiation, the charges migrating in a plane parallel to the electric field. The position along the Z-coordinate or interaction depth can be determined by calculating the rise time according to the method described in this document.
It is therefore possible through the use of non-collecting electrodes arranged within or in proximity to an anode network, to determine the coordinates of an interaction in a detector material in a plane perpendicular to the electric polarization field of the collecting electrodes.
However, and as recalled previously, it is known that the mobility, and/or the life cycle of the charge carriers in some detection materials, is low. Thus, when the interaction of the incident radiation with said detector material occurs beyond a certain distance from the polarization electrodes or collecting electrodes, the charge carriers fail to reach a collecting electrode. Thus, in a hypothetical case such as this, the interaction is not detected. The useful thickness of the detector material, in other words that in which interactions give rise to the genesis of charge carriers collected by a collecting electrode, is therefore limited. In doing this, the detection efficiency of such a device proves limited.